1. Field of the Invention
The present invention relates generally to a method and apparatus for reading multilevel signals from an optical disc and writing multilevel signals to an optical disc. The invention relates to methods and apparatuses for processing signals that are eventually written to and read from an optical disc. These signals produce marks on the optical disc that may vary in both reflectivity and length. The system disclosed provides a method of encoding and decoding the data, correcting for errors, synchronizing the data, controlling the DC content, establishing and recovering a clock signal, establishing and recovering the envelope of the signal, and compensating for signal distortion.
2. Relationship to the Art
In order to increase the capacity and speed of optical data storage systems, multilevel optical recording systems have been developed. It should be noted that in this specification, the term multilevel is used to indicate greater than 2 levels. In a traditional optical recording system, reflectivity of the recording media is modulated between two states. The density of data recorded on an optical recording medium may be increased by modulating the reflectivity of the optical recording medium into more than two states.
One type of optical recording medium that appears to be particularly suitable for multilevel signal modulation is phase change optical material. When a phase change material is heated by a writing laser, the reflectivity of the phase change material may be changed. The change in reflectivity may be controlled by adjusting the amount of heating of the material and the rate at which the material cools. This process is described further in xe2x80x9cLaser-induced crystallization phenomena in GeTe-based alloys. I. Characterization on nucleation and growthxe2x80x9d (J. Appl. Phys. 78 (8), Oct. 15, 1995. p. 4906) by J. H. Coombs, et. al. (hereinafter xe2x80x9cCoombsxe2x80x9d).
After a phase change optical disc has been written, the intensity of a beam of light reflected from the disc is measured so that the multilevel data written to the disc may be recovered. U.S. Pat. No. 5,144,615 entitled APPARATUS AND METHOD FOR RECORDING AND REPRODUCING MULTILEVEL INFORMATION issued to Kobayashi (hereinafter xe2x80x9cKobayashixe2x80x9d) discloses a system for recovering multilevel data from such an optical disc. FIG. 1 is a block diagram illustrating the system disclosed in Kobayashi for recovering such data. Analog data read from a detector is input from a mark length detecting circuit 101 and a reflectivity detecting circuit 102. The outputs of these circuits are sent to an analog-to-digital (A/D) converter 103. The A/D converter 103 includes an n-value circuit which determines the value that the signal corresponds to by comparing the signal to predetermined reference voltages. Subsequently, the n-value signal is converted into a binary signal by binary circuit 405.
While this system discloses the concept of reading a multilevel signal and converting it into a digital signal in a basic sense, no method is disclosed of handling various imperfections in optically read multilevel signals that in fact tend to occur. For example, it is not clear how a clock is recovered for the purpose of precisely detecting mark lengths and no method is disclosed for handling problems that tend to occur in real systems such as amplitude modulation and DC offset of the optically detected signal and noise.
In a conventional two level optical data storage system, information is stored in the lengths of the marks and the spaces between them. So long as the edge of a mark can be detected with enough precision to distinguish between marks that differ in length by a minimum allowed amount, the system can operate reliably. This edge transition between one reflectivity state and another can be detected by setting a threshold value and determining the time when the signal crosses the threshold. Slow amplitude variations that might interfere with this edge detection are removed by AC coupling the photodetector signal before the threshold detection circuit. Mark and space lengths are measured by counting how many clock periods are between the edge transitions. The reader clock periods are synchronized to the mark/space edges, thus ensuring that there are an integral number of clock periods in each mark/space.
In contrast, in a multilevel recording system, it is the amplitude of the signal that carries information. The reader must interpret the data signal to determine the amplitude of the signal at certain times. Therefore, the reader clock must be synchronized to the data stream to ensure that the reader is interpreting the signal at the proper time. Because of the blurring effect of the optics in a reader, the transitions between the different levels do not create sharp edges. It is therefore difficult to synchronize the reader clock to the data stream. A method of precisely aligning a read data stream is needed. Further, a multilevel system is more sensitive to fluctuations in the overall envelope of the data signal. AC coupling alone is not adequate to enable a sufficiently precise determination of the different amplitude signals. Another problem encountered in a multilevel optical disc system is DC compensation.
In order for a multilevel optical read system to reliably record and recover data, a method of handling these sources of error in reading an optical signal is needed.
Accordingly, a system for writing and reading multilevel marks on an optical disc is disclosed. The system includes an error correction encoding and decoding system, modulation and demodulation system, DC control system, amplitude correction circuit, a clock recovery circuit, a write strategy system, a system to focus and track the laser spot on the surface of the disc, a system to rotate the disc, and an interface to a computer system. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium that includes certain types of marks that enable reliable data storage and recovery. Several inventive embodiments of the present invention are described below.
In one embodiment, method is disclosed for reading a multilevel signal from an optical disc. The method includes reading a raw analog data signal from a disc using an optical detector and adjusting the amplitude of the raw analog data signal. A timing signal is recovered from the amplitude adjusted analog data signal and correction is made for amplitude modulation of the raw analog data signal by processing the raw analog data signal and the timing signal.
In another embodiment, a method of reading a multilevel signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector and recovering a timing signal from the raw analog data signal. The analog signal is converted to a digital data signal using an A/D converter. Amplitude modulation of the raw analog data signal is corrected by processing the digital data signal to obtain an amplitude adjusted digital data signal.
In another embodiment, a method of reading a multilevel signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector and correcting for amplitude modulation of the raw analog data signal by processing the raw analog data signal to obtain an amplitude adjusted analog data signal. A timing signal is recovered from the amplitude adjusted analog data signal and the timing signal is used to further correct for amplitude modulation in the amplitude adjusted analog data signal.
In another embodiment, A method of reading a multilevel signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector and correcting for amplitude modulation of the raw analog data signal by processing the raw analog data signal to obtain an amplitude adjusted analog data signal. A timing signal is recovered from the amplitude adjusted analog data signal. The amplitude adjusted analog data signal is converted to a digital data signal. The digital data signal is processed with a fractionally spaced equalizer to obtain an equalized data signal.
In another embodiment, a method of reading a signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector. The raw analog data signal includes an alignment sequence. The alignment sequence is chosen such that the autocorrelation of the alignment sequence has a substantially high value at a single alignment point. The raw analog data signal is converted to a digital data signal. The digital data signal is cross correlated with a stored digital version of the alignment sequence so that the start of a data sequence can be determined.
In another embodiment, a multilevel pattern of marks written to an optical disc includes a preamble and a data block. The preamble includes a timing acquisition sequence of fields, an alignment sequence, a calibration sequence of marks, and an equalizer training section.
In another embodiment, A multilevel pattern of marks written to an optical disc organized into an ECC block includes modulation encoded marks and physical format marks. The modulation encoded marks include an encoded address section and encoded data marks. The physical format block marks include periodic ECC data synch fields, periodic timing fields, periodic AGC fields, and periodic DC control fields.
In another embodiment, a method of recording data on an optical disc includes defining a desired data sequence and deriving a write signal from the desired data sequence using a write strategy. The optical disc is recorded using the write signal and the optical disc is read to obtain a recovered sequence. The recovered sequence is compared to the desired data sequence and the write strategy is adjusted based on the comparison of the recovered sequence to the desired data sequence so that the recovered sequence tends to converge toward the desired data sequence.
In another embodiment, a method of recording data on an optical disc includes defining a desired data sequence and deriving a write signal from the desired data sequence using a write strategy. The optical disc is recorded using the write signal and the optical disc is read to obtain a recovered sequence. The desired data sequence is linearly filtered. The recovered sequence is compared to the linearly filtered desired data sequence and the write strategy is adjusted based on the comparison of the recovered sequence to the linearly filtered desired data sequence so that the recovered sequence tends to converge toward the linearly filtered desired data sequence.
In another embodiment, a multilevel pattern of marks written to an optical disc includes data marks that include more than two levels of data, timing fields occurring periodically between the data marks, and automatic gain control fields occurring periodically between the data marks wherein the automatic gain control fields correspond to a specific level of data.
In another embodiment, a method of writing a multilevel signal to an optical disc includes encoding data by mapping the data onto a plurality of levels including more than two levels, adding synchronization fields to the encoded data, determining a DC level of the encoded data, and adding DC control fields to keep the DC level of the encoded data substantially constant.
In another embodiment, a method of writing a multilevel signal to an optical disc includes determining a raw data sequence having raw data sequence elements and encoding the raw data sequence using a convolutional code to obtain a correlated data sequence. The correlated data sequence has correlated data sequence elements that are a function of more than one element of the raw data sequence. The correlated data sequence is written to the optical disc.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.